A circulation channel, or race track, is usually an upwards open endless basin used for biological treatment, or oxidation, of especially waste water. The waste water is made to flow along the circulation channel and thereby is made to pass different zones in the circulation channel.
During such biological cleaning the waste water is usually purified from nitrogen and biological material by having micro organisms breaking down the biological material into carbon dioxide and water, and by having bacteria transforming the water bound nitrogen to aerial nitrogen. Purified waste water is released into the nature and in the case the water bound nitrogen is not eliminated there is a risk for eutrophication in the natural watercourses, and due to the fact that the biological material is consuming considerable amounts of oxygen watercourses deficient in oxygen are generated if insufficiently purified water is released. The breaking down of the biological material is stimulated by adding large amounts of oxygen to the waste water by means of one or more aeration sectors, and the elimination of the water bound nitrogen takes place in the circulation channel in areas without added oxygen of in separate basins without added oxygen and/or in areas/basins in which the dissolved oxygen level is low enough. Along the circulation channel at least one so-called aerated zone is found and at least one so-called non aerated zone. Thus, the abovementioned aeration sector is arranged in the aerated zone.
In one, or a few, aerated zones along the circulation channel oxygen is supplied to the waste water by means of a gas flow, in the form of large amounts of gas bubbles, that is supplied to the waste water by means of aeration sectors arranged at the bottom of the circulation channel. It is advantageous to provide the gas flow as close to the bottom of the circulation channel as possible in order to maximize the dwell time of the gas in the liquid which promotes transfer of oxygen from the gas to the liquid. The micro organisms in the so-called activated sludge consume the oxygen in order to break down the biological material present in the waste water, as well as possibly for nitrification of inter alfa ammonium nitrate.
Thus, for the treatment of liquid a certain amount of oxygen in the liquid to be purified is required and/or a certain oxygen transfer rate, seen at a definite location in the specific treatment plant, which oxygen level is chosen in order to provide the best conditions possible for the activated sludge to break down the biological material. When the quantity of biological material is large, a large amount of transferred oxygen is required and the treatment plant is operating at a maximum level, and when the quantity of biological material is less, a less amount of transferred oxygen is required and the treatment plant is operating at a mean level, i.e. the need for transferred oxygen, or the oxygen transfer rate to the liquid, varieties along with the varying quantities of oxygen consuming biological material in the liquid.
Flow generating machines/mixer machines are used in circulation channels in order to mix the liquid/waste water in order to obtain an as homogenous liquid mixture as possible, keeping the biological material suspended, as well as generating a liquid flow circulating/flowing along the circulation channel.
The power consumption, and thereby the cost, for operating such a treatment plant is really high, and above all the operation of the aeration arrangement and the flow generating machines are together the most outstanding power consumer at waste water treatment. The power consumption of the aeration arrangement is about ten times greater than the power consumption of the flow generating machine in known treatment plants.
Due to the fact that the aeration arrangement according to prior art for instance was controlled directly based on the dissolved oxygen level in the liquid, the power consumption of the treatment plant was minimized when the gas flow/amount of gas, i.e. the amount of oxygen, supplied to the liquid by the aeration arrangement is adjusted to be as small as possible, i.e. gas flow minimization. This is achieved by minimizing the operational speed/rpm of the blowing machine/compressor of the aeration arrangement, or by means of a valve in the gas conduit extending to the aeration arrangement throttles the gas flow and instead use the gas flow in another aeration sector/basin.
Due to the nevertheless large power consumption every additional decrease of the power consumption/costs for liquid treatment is still of great interest from an economical point of view as well as from an environmental point of view.